In a conventional IDG system, an input shaft connectable to gearbox driven by an aircraft engine is connected to a mechanical differential, the differential having an output connected to drive a generator. A variable speed transmission, such as a hydromechanical transmission, is associated with the mechanical differential and controlled to modify the output of the differential, as required, whereby the input speed to the generator remains constant even though the speed of the input shaft may vary. In conventional IDG systems which provide for engine starting, the generator is replaced by a motor/generator which is driven by a constant frequency source of electrical energy. Mechanical output from the motor/generator is routed through the variable speed transmission and differential to an input/output shaft. In other words, the conventional generating-starting IDG system simply reverses the power path for engine starting.
In the past, IDG's have been modified in many ways to perform under a variety of operating requirements. Following are descriptions of IDG's, all of which have the capacity to start aircraft engines, although methods of doing so may vary greatly.
U.S. Pat. No. 4,315,442 ("'442"), which issued on Feb. 16, 1982 to Cordner, is directed to a hydraulic control system for an aircraft starter/drive mechanism that includes a generator that can be driven as a motor drivingly connected through a differential, which is mutually connected to first and second hydraulic units and to an engine drive shaft during a start mode. The hydraulic control system includes a control valve arrangement, cooperatively coupled to the first and second hydraulic units, and operative in the starting mode to control flow between the hydraulic units, to thereby divide the delivery of rotary power from the motor/generator to the engine drive shaft through the differential and the hydraulic units. In other words, Cordner '442 provides for a starter/generator system which has different drive ratios for start and generate operation. Further, Cordner uses the hydromechanical drive in the start cycle, running the generator as a motor at its synchronous speed while starting. The invention to be described, on the other hand, is directed to exclusion of the hydromechanical transmission during start mode to avoid deleterious viscous drag and friction.
U.S. Pat. No. 4,708,030 ("'030"), which issued on Nov. 24, 1987 to Cordner, is directed to a starter/generator drive useable in an aircraft which must be of the smallest possible size and the lowest possible weight and have maximum reliability. A drive having a multi-speed transmission and a controllable hydro-viscous dissipative clutch can be interposed between an engine and a starter-generator to achieve maximum efficiency in the drive and meets the forgoing objectives. The starter-generator drive has a multi-speed transmission for stepping the input speed from an engine to a hydro-viscous dissipative clutch to provide plural speed ranges of operation and thus limit the amount of slip that occurs within the clutch in each range to provide the constant speed drive of the generator. The dissipative clutch operates in a hydro-viscous manner whereby sudden changes of speed, as the multi-speed transmission shifts, do not vary the torque transmitted to the generator and therefore the speed of the generator. This avoids transients in the output power frequency of the generator during ratio changing of the multi-speed transmission. The starter-generator drive also provides for dissipative engine start utilizing the generator as an electric motor. In other words, Cordner U.S. Pat. No. 4,708,030 provides for an electro-mechanical transmission which has a slip clutch and brake for the purpose of changing the mode of operation. In the start mode, the starter-generator drives through the differential and the multi-speed transmission. The present invention, on the other hand, is designed to perform an engine start in a direct fashion, without involving an intermediate hydromechanical transmission and the friction introduced by inclusion thereof.
U.S. Pat. No. 4,473,752, which issued on Sept. 25, 1984 to Cronin, is directed to a starter/generator machine for starting turbine type aircraft engines. The machine combines an induction motor with a synchronous samarium cobalt generator. In the machine, a rotor-shaped stator is fixed and positioned inside a squirrel-cage induction rotor which has an array of samarium-cobalt magnets attached on the outer diameter thereof. The compound dual machine operates as a starter by using the induction rotor to accelerate the permanent magnet rotor, and thus the aircraft engine via a drive pinion, up to some low synchronous speed, when AC power is applied to the outside stator to lock in the permanent magnet rotor synchronously with the rotating field created in the stator of the synchronous generator. As the speed of the rotor is then increased, the engine speed is also increased via the drive pinion. In a second embodiment disclosed, a cartridge type induction-motor is used to initially start an aircraft engine. The motor includes a gear reduction and disconnect clutch and drives the engine through a splined pinion which in turn drives an engine connected spline. When the speed of the machine is such that the synchronous operation of an outside permanent magnet rotor commences, the clutch is utilized to disconnect the induction motor cartridge, leaving the rotor to drive the drive spline via internal splines. Cronin is, therefore, directed to the construction of an induction motor/synchronous motor-generator to be used for starting engines. Cronin fails to provide for a constant speed drive in the generate mode and does not bypass the constant speed drive in the start mode. Accordingly, Cronin does not provide for one-way clutches to accomplish the above.
U.S. Pat. No. 3,786,696, which issued on Jan. 22, 1974 to Aleem, is directed to a starter-drive for use between an aircraft engine and a generator to transmit power in either direction between the engine and the generator, for driving the engine from the generator in a starting mode, and for driving the generator in a generating mode, including a generator shaft, an engine shaft, a differential for transmitting power from the engine shaft to the generator shaft, a hydrostatic transmission including one hydraulic unit connected for rotation with the generator shaft and a second hydraulic unit connected for rotation with a controlled gear in the differential, a first one-way clutch connecting the second hydraulic unit to drive the engine shaft exclusively through the hydrostatic transmission during starting, a second one-way clutch connecting the engine shaft to the differential to transmit power from the engine to the generator after the engine has started, and means for varying the displacement of one of the hydraulic units to bring the engine shaft up to speed during the starting mode and to add or subtract speed in the differential during the generating mode. Aleem apparently discloses a hydro-mechanical transmission generator drive which has overrunning clutches which cause it to operate as a hydrostatic transmission in the start mode with the generator running at a constant speed. The present invention is specifically directed to the object of bypassing the hydrostatic transmission during engine starting. Therefore, Aleem and the present invention have different objects.
In summary, the present invention is the first to provide a hydromechanical transmission for a motor/generator drive which, through the use of overrunning clutches, allows a start mode which bypasses the hydromechanical transmission and operates the motor/generator as a variable speed synchronous motor by the use of pulse width modulation supplied by a start inverter.